1. Field of the Invention
This invention relates to centrifugal pumps of the pitot tube type, and specifically relates to modification of the pitot tube to extend the life of the pitot tube and to selectively control the performance characteristics of the pitot tube.
2. Statement of the Art
Pitot tubes are used in many different industries for the movement or transport of a fluid under high pressure, and are frequently used in measuring devices. In a particularly relevant application to the present invention, pitot tubes are used in centrifugal pumps to move a liquid through the pump at very high pressure. Examples of pitot tube pumps are disclosed in U.S. Pat. No. 3,776,658 to Erickson; U.S. Pat. No. 3,822,102 to Erickson, et al.; U.S. Pat. No. 4,183,713 to Erickson, et al.; U.S. Pat. No. 4,252,499 to Erickson and U.S. Pat. No. 4,279,571 to Erickson. Conventional pitot tubes are cast from a material of suitably high strength such as, for example, 17-4 stainless steel, 718 Iconel, Stellite, Hastelloy or other similar materials. Pitot tubes are generally cast as a body having a base, an elongated neck portion and a head. A bore is cast in the body of the pitot tube which extends from an opening in the inlet of the pitot tube to an opening in the base of the pitot tube. The pitot tube is typically formed with a shoulder which enables the body to be attached to the centrifugal pump in a manner which aligns the bore of the pitot tube with a discharge tube for discharge of the pumped fluid from the pump.
Pitot tubes in centrifugal pumps generally function to collect fluid which is circulating within the rotary casing of the pump under centrifugal force. The fluid enters the rotary casing of the pump along the axis of rotation and picks up momentum as it passes through the radial vanes of the impeller and into the rotary casing. The fluid maintains its velocity at nearly the rotational speed of the casing. The fluid then moves into the inlet of the pitot tube, through the inner channel of the pitot tube and out of the pump by way of a discharge tube. The inlet of the pitot tube is usually located near the periphery of the rotary casing. This is where the pressure and rotational velocity of the fluid are greatest. The inlet of the pitot tube, which is usually stationary, is impacted by the fluid as it circulates in the rotary casing. Under some operating conditions, cavitation can occur at the inlet of the pitot tube which essentially disintegrates and/or erodes the metal. When the fluid contains particulate matter or solids, the impact of the fluid and solids on the pitot tube can also be very damaging. Under normal operating conditions, therefore, the pitot tube may be damaged, head loss may be experienced and/or any number of other deleterious effects may limit the function and operation of the pitot tube pump.
Others in the art have recognized that damage can occur to the pitot tube under such conditions as described. Thus, modifications of the pitot tube, including the inlet, have been suggested in the literature to reduce damage to the pitot tube. Examples of such modifications are disclosed in U.S. Pat. No. 3,999,881 to Crichlow, U.S. Pat. No. 4,264,269 to Erickson, et al., and U.S. Pat. No. 4,674,950 to Erickson.
While modifications of the design of the pitot tube as previously described in the art can be beneficial, wear and damage to the pitot tube still occurs and eventually the pump must be taken off-line for repair or replacement of the pitot tube. Taking the pump off-line in an operation is not only costly and time-consuming, but the repairs which must be effected on the pump are usually considerable since the entire pitot tube assembly of the pump must be replaced. Further, because conventional pitot tubes are cast, the shape and dimension of the inlet of the pitot tube is predetermined and sized to accommodate the parameters of an average application. One disadvantage of conventional cast pitot tubes, therefore, is that one size or dimension of the inlet and the opening is not suitable for all types or conditions of operation. While some modifications may be made to the inlet and opening by machining the pitot tube after casting, the types of modifications that can be made are limited. For example, it has been recognized that fluid striking the inlet of the pitot tube can cause cavitation or erosion damage to the inlet such that the inlet and surrounding areas of the head of the pitot tube are eventually disintegrated or eroded away. To ameliorate some of the damage caused by cavitation and other erosive modalities, the inlet of a conventional pitot tube may be modified or finished to provide a sloping shoulder to deflect fluid striking the inlet. However, prolonged use and exposure of a pitot tube so modified still leads to damage from cavitation or other destructive forces and the pitot tube eventually requires replacement. With conventional cast pitot tubes, the entire body of the pitot tube must be removed, which requires complete dismantling of the pump.
Therefore, it would be advantageous in the art to provide a pitot tube which is better able to withstand wear and damage wrought by the type of fluid being processed through the pump, which enables the pump to be more easily and economically manufactured, and which is easily and quickly repaired reducing the extent to which the pump must be dismantled and refurbished, thereby reducing costly repairs and lost production time. It is would be advantageous to provide a means for selectively modifying the inlet of a pitot tube to present an inlet which is selectively shaped to resist damage, which is replaceable after sustaining damage and which is modifiable to meet the specific requirements of a pumping application.